“Brake-by-wire” brake systems are being used ever more widely in motor vehicle engineering. Such brake systems often comprise not only a brake master cylinder that can be actuated by the vehicle driver but also an electrically controllable pressure supply device, by means of which actuation of the wheel brakes, either directly or via the brake master cylinder, takes place in the “brake-by-wire” operating mode. In order to give the vehicle driver a pleasant pedal feel in the “brake-by-wire” operating mode, such brake systems generally have a brake-pedal-feel simulator, which is in operative connection with the brake master cylinder, for example. To control the brake system, a setpoint device is provided, the device evaluating the electrical signals from one or more sensors for detecting the driver's braking intention (actuating intention), for example, in order to determine a setpoint value for control of the pressure supply device. In these brake systems, however, the pressure supply device can also be actuated without active intervention by the driver on the basis of electronic signals. These electronic signals can be output by an electronic stability program (ESC) or a headway control or adaptive cruise control (ACC) systems, for example, the setpoint device thus determining a setpoint value for control of the pressure supply device from these signals.
A method for controlling an electrohydraulic brake system for motor vehicles having an electronically controllable pressure supply device, which is connected to hydraulically actuable wheel brakes, is described in DE 10 2011 076 675 A1. The pressure supply device comprises a cylinder-piston arrangement with a hydraulic pressure space, the piston of which can be moved relative to a rest position by an electromechanical actuator. For closed-loop pressure control, an actual value for the inlet pressure and a setpoint value for the inlet pressure are determined, these being fed as input variables to a controller, which sets the corresponding setpoint value for the inlet pressure in the hydraulic pressure space.
In WO 2012/010475 A1, the proposal is that the pressure in the hydraulic pressure space of the pressure supply device should correspond to the respective highest setpoint wheel pressure of the wheel brakes during antilock control, wherein at least one wheel brake with the highest setpoint wheel pressure is connected to the hydraulic pressure space via the open inlet valve to give complete pressure equalization between the wheel brake and the pressure space. The system pressure of the pressure supply device is set to the highest setpoint wheel pressure by means of pure closed-loop pressure control.
Among the disadvantages of the closed-loop pressure control of the system/inlet pressure of the pressure supply device as described in WO 2012/010475 A1 and DE 10 2011 076 675 A1 is the fact that, in the case of a desired pressure buildup at one wheel brake, there can be a brief, unwanted drop in the system/inlet pressure and hence an unwanted pressure reduction at another wheel brake when the associated inlet valve of the first wheel brake is opened, before the system or inlet pressure drop can be balanced out again by the closed-loop pressure control system. Accurate and stable setting of the pressure at the wheel brakes is thus not always guaranteed.
It is therefore the object of the present invention to provide a method for operating an electrohydraulic brake system for motor vehicles and to provide a brake system, which method and system allow improved, wheel-specific setting of the pressure at the wheel brakes. In this context, the setting of the pressure at the wheel brakes should be as precise but also as comfortable as possible.
According to the invention, this object is achieved by a method and by a brake system as described herein.